The present invention relates to a control system for a supercharged internal combustion engine, and more particularly to a control system for controlling an internal combustion engine having a supercharger associated with a rotary electric machine.
Internal combustion engines on some recent motor vehicles are equipped with a supercharger comprising a turbine driven by the energy of an exhaust gas emitted from the engine and a compressor coupled to the turbine for charging air into the engine cylinders to burn the fuel efficiently.
There has been proposed a rotary electric machine connected to the turbine shaft of such a supercharger, the rotary electric machine being operable as either an electric motor or an electric generator depending on the operating condition of the engine (see Japanese Patent Application No. 58(1983)-51559).
One widely used type of motor vehicle brake is known as an exhaust brake which brakes the vehicle with increased engine braking by closing the exhaust pipe of a diesel engine, for example.
As shown in FIG. 4 of the accompanying drawings, when a supercharged internal combustion engine is operated in medium and high speed ranges, the turbine of the supercharger rotates at a high speed to increase the pressure of the supercharged air since the exhaust energy emitted from the engine is large. Therefore, a torque curve Tb plotted in the medium and high engine speeds is higher than a torque curve Ta plotted when the engine is not supercharged. While the engine is being operated in a low speed range, no sufficient boost pressure is generated even if the supercharger is driven since the exhaust energy is small. Consequently, the engine torque is generally low when the engine speed is low.
In order to increase the engine torque at low engine speeds, it has been practiced to reduce the opening of the nozzle which supplies the exhaust gas to the turbine, thereby increasing the pressure on the turbine. However, the device for varying the nozzle opening is costly, and it is difficult to achieve a desired level of durability of the turbine which is subject to high-temperature exhaust gases.
The exhaust brake arrangement as referred to above requires a valve of a high sealing capability to be installed for closing the exhaust pipe, and is also disadvantageous since there is a limitation on the range of materials available for the valve that is held in contact with the high-temperature exhaust gas, and the shaft for opening and closing the valve must be lubricated.
Another problem of the exhaust brake system is that the exhaust energy is not effectively utilized since it only increases the engine braking action by simply closing the exhaust pipe to raise the back pressure.